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Re: rfc 1918?

  • From: Valdis.Kletnieks
  • Date: Thu Feb 22 22:20:52 2001

On Thu, 22 Feb 2001 19:12:14 EST, Mark Radabaugh <>  said:
> At our egress points the filters are fairly short -- they allow only traffic
> with our IP source addresses to leave.  This was my interpretation of the RFC's.

Thank you.  The rest of the network appreciates you doing your part.

> Some in this discussion seem to be saying that we should also filter for RFC1918
> destinations.  Am I reading this correctly?

That's probably optional.  But if you have the router resources to do it.
every little bit helps.  You probably should filter and log it, to find out
why one of your hosts is trying to send to a 1918 address outside your
site - if you're not using 1918 space, it shouldn't happen, and if you ARE
using it, the packet should have ended up inside your net, not on your
border router.

In either case, if a packet is trying to leave your net bound for a 1918
destination, something is probably seriously wrong(*).

(*) I'll leave ICMP replies from 1918-addressed P2P links out for the moment ;)
>  I can see that packets destined for RFC1918 addresses will leave our network
> (due to default routes) but are promptly dropped at the first BGP speaking
> router they encounter.  Is it worth the extra router processing time to check
> all outgoing packet destinations as well?  I can't see where this extra
> filtering is worth the trouble.

There's 2 main classes of "next router":

1) You don't filter, but it goes to the OTHER end of the link and promptly
gets stomped by a fascist filter that refuses to accept any source address
that's outside the adddress block that's supposed to be at your end.

2) You don't filter, they don't filter either, because they actually USE
1918 space for their own stuff, so your packets wtih 1918 source addresses
and real destination addresses manage to go a LONG way before hitting
anything that will stop them (as the original poster showed, the packets
could actually *arrive* at the destination, with no way to reply).

Remember - this filtering often can't be done on core routers due to
performance issues, so if it doesn't get done at the border routers
it probably won't happen...

				Valdis Kletnieks
				Operating Systems Analyst
				Virginia Tech

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